The mode we have just described of 'setting a sponge' is somewhat varied in ordinary practice. A process commonly employed in home-baking is to mix the yeast smoothly with a small quantity of gently-warmed water (sometimes a little milk is added, which promotes fermentation), and pour the compound into a hole made in the middle of the flour. A portion of the flour is next stirred into the liquid and a further portion strewed over the top. When left in a warm place, fermentation goes on, and the bubbles raise the sponge through the top coating of flour. The whole is then kneaded together with the addition of the requisite quantity of water, and left once more to rise. The kneading is again repeated, after which the dough is formed into loaves, and these are left till they have swelled to double their primitive size, when they are put into the oven. The object in this plan is to be certain of a good fermentation in a stage of the process when there is still time to add more yeast, that is before the kneading is completed. Those who prepare bread in large quantities for sale, carry out the principle of the 'sponge' or leaven in a still more complicated fashion, the flour and water being added, with additional yeast, in successive relays, at intervals of several hours. Instead of the addition of milk to the yeast, a cheaper substitute is said to be extensively used by bakers in setting the first sponge. This is a thickish paste made by mashing boiled potatoes with water, which from the softened condition of its starchy contents is more readily attacked by the ferment than pure flour.

Yeast is known to most persons merely as a creamy liquid, or in the German yeast as a tough semi-solid matter, which softens when placed in water. When the dough, through its influence, 'rises' into a spongy mass, it may be compared to a species of solidified froth. Froth arises from the entanglement of particles of air or other gaseous matter in a more or less viscid liquid. The air confined in 'soda-water' is a gas dissolved in pure water; when the pressure is removed it escapes very rapidly, so that no permanent froth is produced. In bottled beer and ginger beer, the carbonic acid does not escape so freely, since the liquids possess a certain viscidity which retains the gas in bubbles. We cannot blow bubbles from pure water, but soap and water is sufficiently tenacious for the purpose. The cavities or bubbles in dough are produced exactly in the same way as those of the froth of bottled beer or other effervescing liquid; but two circumstances concur in bread to render them permanent; first, the fact that they are slowly formed; secondly, that they are generated in a substance which, while it is soft enough to allow the bubbles to expand, is tough enough to retain them. One of the main objections to what is called 'unfermented bread,' in which the

lightness

lightness is produced by simple effervescence, arises from the sudden liberation of the gas on mixing the acidulated water with the flour containing a carbonated alkali. It becomes necessary to place the bread immediately in the oven, or its spongy character disappears.

It will surprise many persons to be told that yeast is a plant. It belongs to the class of Fungi, and in accordance with the general habit of its kind it differs from the green forms of vegetable life by feeding upon organic substances. The vegetative structures, mostly colourless and often undistinguishable without the aid of the microscope, are in many cases extraordinarily developed when their presence is hardly suspected except by the botanist. Gardeners are aware that the productiveness of their mushroom-beds is dependent on the healthy development of a mass of 'spawn,' of which the mushrooms are the fruit; but many persons are ignorant that the toad-stools upon rotten wood are the mere indices of an invisible but widely-spreading spawn carrying destruction in the form of 'dry-rot' as it extends itself among the fibres of the wood. Again, the appearance of moulds and mildews upon, preserved vegetable substances or liquids is an index that the mischief is far advanced; for these are but the fruits of the fungi, produced in most cases only after the vegetative structure (mother,' flocculent clouds, and the like) has extensively spread.

6

The Yeast-plant represents one condition of a species of fungus remarkable for the diversity of forms it exhibits, its wide, nay, universal distribution, and the magnitude of the effects, sometimes beneficial, sometimes mischievous, which it is capable of producing. The forms in which it is familiar to most persons, although its nature may be unsuspected, are yeast, the gelatinous vinegar-plant, the mother' of vinegar and many decomposing vegetable infusions, and the common blue or green 'mould' (Penicillium glaucum) which occurs everywhere on sour paste, decaying fruits, and in general on all dead organic matters exposed to combined moisture and moderate heat. Yeast and the Vinegar-plant are the forms in which it vegetates, under various circumstances, when well supplied with food. Mildew is its fruit, formed on the surfaces exposed to air, at certain epochs, like the flowers and seeds of the higher plants, to enable it to diffuse itself, which it does most effectually, for the microscopic germs, invisible singly to the unaided sight, are produced in myriads, and are so diminutive that ordinary motes floating in the atmosphere are large in comparison. When we know that the pollen-dust of fir-trees, which consists of particles upwards of six times the diameter of the spores of this mildew,

sometimes

sometimes falls upon ships distant many hundred miles from land, we cannot marvel at the ubiquitous character of the yeastfungus.

Yeast, when examined under the microscope, is found to consist of globular vesicles, about 1-2500th of an inch in diameter when full-grown. They are multiplied by little vesicles budding out from the sides of the parent. These soon acquire an equal size, and repeat the reproduction either while attached to the parent globule or after separating from it. The multiplication goes on to an indefinite extent with a fitting supply of food, and at a moderately warm temperature (70°-90° Fahr.). The vesicles are nourished by sucking in a portion of the organic liquid in which they exist, decomposing this chemically, and either actually giving off or causing the separation at their outer surface, of carbonic acid in the form of gas, which appears as bubbles in the liquid. This, however, is not all. Some phenomena accompany the growth of yeast which involve one of the most abstruse and, at present, obscure questions of modern chemistry. The Yeast-plant is a wasteful feeder. Not only does it decompose so much of the liquid as it requires for its own nutrition, but it produces a similar decomposition in the liquid around it, which is referred by chemists to the same unexplained force by which many inorganic substances cause the combination or separation of substances without themselves undergoing alteration. An example is furnished by the action of spongy platinum upon a mixture of hydrogen and oxygen gases, causing them to unite and produce water. This contact-action is at present a stumblingblock to natural philosophers, many of whom are earnestly endeavouring to surmount it, and, when this is accomplished, they will probably secure a 'point of vantage' whence they will gain an insight into a multitude of unexplained phenomena of vegetable and animal chemistry.

It must suffice at present for us to know that the action does occur, and exercises most important effects in decomposing organic matters for its own nutrition, and propagating this decomposition throughout its neighbourhood. This is the phenomenon which is called 'fermentation,' and which takes place in liquids containing starch, dextrine, or sugar, when a certain amount of albuminous matter is present. Infusion of malt, to which yeast is added, ferments, when kept at a certain temperature, into beer. The dextrine and sugar extracted from the malt by the water, increased during the fermentation by a further conversion of the starch remaining in the malt, pass by degrees into the form of alcohol, carbonic acid gas being set free. This constitutes alcoholic fermentation. If the beer, or fermented liquid

containing

containing a small percentage of alcohol, is exposed freely to the air, the fermentation advances into another stage or form: the alcohol disappears, and vinegar is found; this is the acetous fermentation, which takes place when beer turns sour. What are called lactic and butyric fermentations may occur in the same materials under certain conditions. It is not uncommon to find lactic acid abundantly formed in the refuse grains' of breweries in hot weather; but the lactic fermentation is still more readily generated in milk, which may be turned' by yeast. The lactic acid produced unites with the albuminous matter or caseine to form the curd. The butyric fermentation occurs frequently in butter and cheese and other putrefying organic substances, and communicates a peculiarly offensive taste and odour. All these fermentations may arise in the process of panification, or the converting of the paste of flour and water into raised dough. The bubbles of carbonic acid are formed, as in the case of beer, at the expense of a certain portion of the starch and albuminous matters in the flour. In making bread the alcoholic fermentation is that which is desired; and part of the skill of the baker is to allow it to continue till it has gone far enough, and to stop it before it has gone too far. The heat of the oven puts an end to the process. The acetous fermentation produces vinegar, which has a solvent action upon the gluten, and diminishes the consistence of the bread; and the lactic fermentation injures both the flavour and colour, while neither of them assists in raising the bread. It is a well-known fact, that sour milk or butter added to dough, left to rise in warm weather, often renders it uneatable. The sour flavour of the Continental black bread is produced by lactic fermentation. The cheesy odour which is evolved from German yeast,' when it has been kept until 'putrid,' affords an illustration of the connexion of the lactic and butyric fermentation with panification. It appears from recent investigations that the colour of ordinary brown bread is not so much dependent on the colouring matter of the bran as upon the nature of the fermentation in dough containing meal derived from the coats of the wheat. There is then an evolution of ammonia, and a darkcoloured substance is formed related to the black products of decayed vegetable matter, to which garden mould, &c., owe their colour.

The alcoholic fermentation resolves part of the material into spirit, which in the baking is driven off in the form of vapour. Many persons may remember the attempts made in the metropolis about twenty years ago by a Patent Bread Company to save this alcohol by condensing the steam in a separate chamber, and distilling the spirit. The plan failed, for the spirit refused to be

saved, and many thousand pounds were sunk in the scheme. If our memory serves us rightly, the bread was dry and disagreeable; at all events, the popular prejudice was in favour of 'bread with the gin in it,' for such was the cry raised at the time by interest and believed by ignorance. Dr. Ödling states that many calculations have been made to show the quantity of alcohol produced in the process of bread-making, and that the amount of proof spirit is estimated at less than one per cent. of the flour. Yet the total, he adds, is so enormous that the bread annually consumed in London yields 300,000 gallons of spirit, all of which escapes into the atmosphere.

The changes effected by the fermentation and the action of heat in baking, may be seen from the following analyses of 100 parts of flour and 100 parts of dried bread :

In the course of the fermentation starch is converted into dextrine and sugar, and part of these products are consumed in the alcoholic fermentation. A portion also of the gluten is destroyed.

The

The quantity of water taken up by the dough and retained in the bread forms an important element when bread is sold by weight. The average quantity found by Messrs. Lawes and Gilbert agreed with the estimates of Dr. Maclagan and the French chemists. These authorities rate the percentage of water in ordinary baker's bread at from 36 to 38 per cent. Professor Johnston calculated that it amounted to 44 per cent. in homemade, and to from 50 to 51 per cent. in baker's bread. latter figures are probably too high. The power to take up and retain a large quantity of water indicates what bakers call 'strength.' Theoretically, a sack of good flour (weighing 280 lbs.) should yield 95 four-pound or quartern loaves; but the quantity of loaves obtained will be more or less in different cases, according to the character of the flour and the skill of the baker. This subject is by no means clearly understood at present, but we gather from the statements of various investigators the following particulars. The 'strength' of a fine description of flour appears to depend upon the high percentage of its gluten, which absorbs water freely, and gives greater consistence to the dough. The whiter flours are commonly deficient in nitrogenous matter. On the other hand, the finer products of hard and more nitro

genous